a) Field of the Invention
This invention relates to an anti-skid braking method suited for application to brakes of an automotive vehicle, and especially to an anti-skid braking method which assures proper control of braking force during running on a rough road. This invention is also concerned with an anti-skid braking system suited for the practice of the anti-skid braking method.
b) Description of the Related Art
An anti-skid braking method is known, which upon braking on a low-.mu. road such as a rain-wet road, can prevent slipping of wheels, retain steerability and allow a vehicle to stop in a short stopping distance. According to this braking method, the revolution speed of each wheel is detected to determine the speed of the wheel. Based on the difference between the speed of the wheel and a reference speed of the vehicle body, the slip ratio of the wheel is determined. Braking pressure for the wheel is then increased or decreased so that the slip ratio can be maintained near an optimal slip ratio at which the coefficient of friction of the wheel becomes the greatest.
When a vehicle is running on a rough road, it is necessary to control braking pressure in accordance with the roughness of the road. According to the conventional anti-skid braking method, a wheel acceleration (i.e., a vibration component of a wheel speed) is detected. Depending on the magnitude of the vibration component, it is then determined whether a road on which a vehicle is running is rough. When the road is determined to be rough, rough road correction of a slip ratio is then conducted.
When detection of a rough road is effected directly from a vibration component of a wheel speed as described above, the vibration component of the wheel speed may not always adequately reflect the roughness because the vibration component itself is obtained as a result of the anti-skid brake control. Especially, when the slip ratio of each front wheel is increased to 100% upon detection of a rough road with a view to improving the braking force, the wheel speed drops to zero (0) thereby making it no longer possible to detect any rough road.